1. Field of the Invention
This invention relates to a novel and useful resist ink composition, and more particularly relates to a liquid resist ink composition developable with an alkaline aqueous solution, which excels in a photosetting property, resistance to heat, resistance to solvents, and resistance to acids and particularly suits the production of printed circuits boards for household appliances and printed circuit boards for industrial appliances.
2. Description of the Prior Art
Heretofore, as means of forming resist patterns on various printed circuit boards for household and industrial appliances, the screen printing method has been prevalently adopted. The screen printing method by nature has low resolution. When screen printing is carried out with a screen printing ink of relatively high viscosity, such adverse phenomena as breaks, screen marks, and pinholes occur in the produced patterns. When screen printing is carried out with a screen printing ink of relatively low viscosity, such adverse phenomena as breeds, smudges, and sags ensue. Because of these defects, the screen printing can no longer keep abreast with the recent trend of printed circuit boards toward increasing density.
To solve these problems, dry film type photoresists and liquid developable resist inks have been devised. To cite an example of dry film type photoresists, a dry film grade photo-sensitive resin composition which comprises urethane di(meth)acrylate, a linear high molecular compound possessing a glass transition point in a specific range, and a sensitizer is disclosed in the specification of Japanese Patent Application Laid-open SHO 57(1982)-55,914. Generally a photoresist of the dry film type, however, tends to produce bubbles during thermocompression bonding, betrays imperfection in thermal resistance and fast adhesiveness, and poses problems such as high price. To cite examples of liquid developable resist inks, UK Patent Application GB 2032939 A published May 14, 1980 discloses photopolymerizable coating compositions which comprise a solid or semi-solid reaction product of a polyepoxide and an ethylenically unsaturated carboxylic acid, an inert inorganic filler, a photopolymerization initiator, and a volatile organic solvent. The specification of Japanese Patent Application Laid-open SHO 58(1983)-62,636 (corresponding to Swiss Patent Application No. 6007/81-1 filed September 17, 1981) discloses a coating composition having a fine granular filler dispersed in a solution of a photosensitive epoxy resin mixed with a curing agent and possessing a viscosity in the range of 200 to 700 mPas so as to suit the curtain coating method. Since the liquid resist inks disclosed in the patent literatures mentioned above and currently offered in the market require use of organic solvents as their developing agents, they pose a problem of air pollution and, besides a problem of expensive solvents, suffer from imperfection in resistance to solvents and resistance to acids. A method which uses an alkaline aqueous solution as a developing liquid in the place of an organic solvent is disclosed in the specification of Japanese Patent Application SHO 57(1982) 164,595 (corresponding to U.S. Ser. No. 225810 filed January 16, 1981). The method for the production of a printed circuit disclosed by this patent literature, unlike the conventional method which produces a film by applying a liquid polymer on a circuit blank and drying the applied liquid polymer, is a unique method which comprises causing the applied liquid polymer in its undried state to be exposed to light through a given resist pattern and cured in that pattern and then removing the uncured liquid polymer. Since this method uses a liquid prepolymer containing many reactive monomers, it is inferior to the conventional film-forming method in such characteristics as resistance to acids and resistance to chemicals. Further, since this method does not form a film by drying, it requires to keep the surface of the applied liquid polymer at a distance from the resist pattern film. This separation impairs the resolution and necessitates provision of a special device and a consequent addition to the cost of production. The disclosures of the various patent applications mentioned above are incorporated herein for the sake of reference.